This invention relates to a color display device provided with a color display tube having an electron gun, a display window opposite the electron gun, and a deflection unit positioned on the outer side of the color display tube between the electron gun and the display window. The electron gun comprises a first focusing electrode, a second focusing electrode and a final electrode, as viewed in the direction from the electron gun to the display window, to which electrodes voltages are applied during operation. The voltage applied to the second focusing electrode is a dynamic voltage, the first focusing electrode and the second focusing electrode forming a quadruple lens system. The electron gun generates, during operation, electron beams which are deflected by a line and a frame deflection field generated by the deflection unit to scan the entire display window.
A color display device as described in the opening paragraph is disclosed in U.S. Pat. No. 4,814,670. The electron gun according to this patent is provided with two focusing electrodes. During operation the first focusing electrode is driven by a constant voltage, while the second focusing electrode is driven by a dynamic voltage.
The geometry of opposing apertures in the first and the second focusing electrode forms a quadrupole lens. The dynamically varying voltage on this electrode causes the quadrupole lens to change dynamically. Furthermore, the main lens formed by the second focusing electrode and the final electrode also varies in a dynamic way. An electron gun of this type counteracts the astigmatism caused by the deflection field and reduces the vertical spot size on the periphery of the screen.
This type of electron gun is known as DAF (Dynamic Astigmatism and Focus) gun, indicating that the dynamic voltage on the second focusing electrode varies both the quadrupole lensxe2x80x94dynamic astigmatismxe2x80x94and the main lensxe2x80x94dynamic focusxe2x80x94of the electron gun.
In practice, however, the colour display device disclosed in U.S. Pat. No. 4,814,670 has some limitations. Especially in colour display tubes with a real-flat screen or with a large deflection angle, it appears that the amount of astigmatism that originates from the deflection unit and has to be counteracted by the electron gun is rather large. This leads to a deterioration of the focus performance of the color display device.
It is in the object of the invention to provide a color display device of the kind described in the opening paragraph, which has a significant improvement of focus performance by overcoming the limitations of the color display device disclosed in U.S. Pat. No. 4,814,670.
According to the present invention, this object is realized with a colour display device, which is improved as compared with the device described in the opening paragraph, and is characterized in that the voltage applied to the first focusing electrode is a dynamic voltage.
The invention is based on the recognition that, for a well-focused picture all over the screen, the dynamic varying quadrupole lens and the dynamic varying main lens have to compensate each other in the direction of the line deflection through the range in which the voltage on the second focusing electrode is changed. This compensation is required because a self-converging deflection field is applied in most colour display tubes. This means that the three electron beams are converged on the entire display window due to the action of the deflection unit. Consequently, in the direction of the line deflection, the electron beams are focused on the entire display window. A change in the dynamic voltage on the second focusing electrode may not lead to a defocusing effect.
Due to the different lens characteristics of a quadrupole lens and a main lens, it is not possible to have a good compensation of the lens action in the direction of the line deflection between the quadrupole lens and the main lens if the dynamic voltage which is required for vertically focusing the electron beam on the screen becomes too large. This is especially the case when the colour display tube is provided with a real-flat screen or when the deflection angle is increased. Both these examples lead to a higher astigmatic component in the deflection unit, which has to be counteracted by the DAF gun.
In a prior art DAF gun, two lensesxe2x80x94the quadrupole lens and the main lensxe2x80x94are varied dynamically by using only one dynamic voltage, namely on the second focusing electrode. In practice, these two lenses do not exactly compensate each other in the direction of the line deflection. The difference in lens action between the quadrupole lens and the main lens that remains when the voltage on the second focusing electrode is varied can be compensated for by also dynamically varying the voltage on the first focusing electrode.
In a preferred embodiment, the dynamic voltage applied to the first focusing electrode varies synchronously with the line deflection field.
In a DAF gun, the dynamically varying quadrupole and main lens counteract the astigmatism that the deflection field introduces and which astigmatism leads to a worse focus performance, that is, a larger spot size on the display window. Due to the self-converging character of the deflection field, the effect on the focus performance is largest in the direction of the line deflection. In first order, the line deflection is a linear function of the line deflection field, which is driven by a sawtooth-shaped voltage. As a consequence, the largest gain in focus performance is achieved if the dynamic voltage is varied as a function of the line deflection field.
In a further preferred embodiment, the dynamic voltage applied to the first focusing electrode varies synchronously with the frame deflection field.
Although the effect of the astigmatism introduced by the self-converging deflection field much smaller is in the frame direction than in the line direction, the focus performance is further improved by adding a component to the dynamic voltage which is varied as a function of the frame deflection.
If the dynamic voltage varies with both the line and the frame deflection, the dynamic voltage is the sum of a component which varies synchronously with the line deflection field and of a component that varies synchronously with the frame deflection field. This is what is understood to be synchronous in both the line and the frame direction.
In another embodiment, the dynamic voltage applied to the first focusing electrode varies substantially parabolically as a function of the line deflection field. When no dynamic voltage is applied, the spots on the display window show a large dimension in the frame direction. In principle, in a colour display tube with a self-converging deflection field, the spot is in focus in the line direction. It appears that the dimension of the spot in the frame direction can be strongly reduced by applying a parabolically shaped focusing voltage. Another advantage of a parabolically shaped focusing voltage is that such a shape is easy to realize. The deflection field is mostly driven with a sawtooth-shaped voltage, that is a linear function of the deflection, and a parabolic voltage can be derived from this by simply integrating the deflection voltage. This argument holds for the line and the frame direction.
In a further embodiment, the dynamic voltage applied to the first focusing electrode varies substantially parabolically as a function of the frame deflection field.
This gives a further improvement of the focus performance because the spot size in the frame direction is now also reduced when the electron beam is deflected in the frame direction.
In another embodiment, the dynamic voltage applied to the first focusing electrode has a value which comprises a fourth-order term as a function of the line deflection field.
A fourth-order term ensures that the correction for the areas close to the edge of the screen better fits the amount prescribed by the astigmatism of the deflection unit. This leads to a focus performance which is even better as compared with the situation where only parabolic corrections are used.